Nickel cadmium storage batteries, having excellent characteristics such as good high-rate discharge characteristics and long charge/discharge cycle life, find extensive application ranging from commercial small-sized appliances to space development, although the currently dominant secondary batteries are lithium secondary batteries and nickel-metal hydride storage batteries.
In nickel cadmium storage batteries, however, a negative electrode active material is repeatedly dissolved and precipitated with charge and discharge. With this repetition, a phenomenon called “migration”, in which cadmium is precipitated to accumulate in pores of a separator, proceeds. The migration occasionally causes short-circuit of a positive electrode and a negative electrode.
Such migration occurs more remarkably particularly in batteries comprising a paste-type negative electrode, which enables low-cost production of high energy-density batteries and has therefore become mainstream in recent years, than in batteries comprising a conventional sintered-type negative electrode. This is ascribed to the fact that the sintered-type negative electrode has an active material carried in a matrix of sintered metal while the paste-type negative electrode uses an organic material for binding active material particles and has no matrix. Since such an organic material is subject to oxidation and decomposition as the battery charge/discharge cycle proceeds, it becomes unable to perform its function of holding the active material particles in the electrode plate. This is the main cause of the migration. Also, the migration is accelerated when the active material is filled into the negative electrode at a higher density in order to heighten the battery capacity. This is because the total change of the active material volume in charge and discharge becomes large.
In an attempt to prevent the short-circuit caused by the migration, Japanese Laid-Open Patent Publication No. Hei 5-283067 proposes formation of a mixed resin layer comprising polyvinyl pyrrolidone and polyvinyl alcohol on the surface of a negative electrode. This can prevent diffusion of cadmium, which has dissolved into an electrolyte in charge/discharge reactions, into a separator to some extent.
However, this related art needs to form a thick resin layer on the surface of the negative electrode in order to fully prevent the short-circuit caused by the migration. Formation of such a thick non-conductive film on the negative electrode surface becomes a problem since it lowers the high-rate discharge characteristics of the battery and hinders absorption of oxygen gas in the overcharged battery.